Fix few style violations in u-boot ports:

• GMAKE -> MAKE_CMD, MAKE_CMD is always set to required make implementation
• CP -> INSTALL_DATA, INSTALL_DATA makes sure files are always installed with proper permissions
• sysutils/u-boot-rpi: include missed files installed in STAGEDIR in PLIST_FILES

PR: 210982
Submitted by: myself
Approved by: maintainers timeout